The long-wave infrared region is the largest continuous IR transmittance window in the Earth's atmosphere. Military aircraft, for example, use the IR communication window via known communication devices having IR sensors. One of the components involved in IR sensors is a primary lens.
An IR primary lens is an IR transmissive structure. An IR transmissive structure transmits IR energy of wavelengths between about 0.1 microns and 20 microns, preferably between 1 and 15 microns, and most preferably between 2 and 12 microns. One material that exhibits IR transmissive characteristics and is commonly used as an IR lens is germanium. Germanium is a suitable material because it has a singular, or binary crystalline structure that is essentially transparent in the IR spectrum.
The shape of the IR primary lens can be aspheric. The parabolic shape of an aspheric lens is ideal for manipulating the focal point of the IR energy waves. Aspheric germanium lenses are normally manufactured by diamond point turning, a very costly and time consuming process.
Salt optics, such as those derived from alkali halides, have the potential of exhibiting improved optical performance over conventional infrared optical materials, such as silicon, germanium, and other materials. Improved performance may include, but is not limited to: increased transmittance, low temperature moldability, reduced cost, and a lower refractive index, which relaxes the requirements for antireflective coatings. However, salt optics are susceptible to degradation when exposed to moisture. For instance, the useful life of the optic may be shortened via exposure to an environment containing an elevated level of moisture, such as an environment with high humidity.
More specifically, most types of salts, when left exposed in high-moisture environments (i.e. those of about >30% relative humidity) erode rapidly. This may result in degraded optical performance or failure. Thus, while salt optics transmit infrared wavelengths over a range of about 0.2 microns to >20 microns, they must presently be employed in moisture controlled environments.